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Roof Sheathing

Roof Sheathing... Re-roofing in the near future

The basic steps required to achieve a safer, more wind resistant roof deck are:

Remove the old shingles and underlying building paper to expose the roof deck.

Inspect the roof deck to determine if parts of it are rotted, delaminated, warped or structurally unsound. If so, replace these sections with similar materials of the same thickness.

Check the nailing of the roof deck to the rafters or trusses that support the deck.

For wood plank decks, the existing nailing has proven to be adequate (If at least two nails were installed every time one of the planks crossed a rafter or truss) so you should not need to re-nail the deck. For wood panel sheathing (plywood or Oriented Strand Board, OSB) nail sizes and nail spacing commonly used, particularly along trusses and rafters in the middle of the panels have been inadequate to provide the needed resistance to uplift in hurricanes. Staples have been found by both engineering testing and hurricane experience to not hold roof sheathing down very well.

Nails and staples as seen from top of roof sheathing – The ply-wood was probably initially nailed in place in the corners and edges and then staples were added.

Missing panels from the middle of roof indicate poor nailing of sheathing.

Comparison of smooth shank and ring-shank nails.

To simplify retrofits throughout the hurricane prone regions of the country, we recommend the installation of 8d ring-shank nails at 6” spacing along panel edges and all the intermediate framing members across the entire roof deck.

Figure above shows the nailing pattern you would like to have holding the roof sheathing down when a hurricane strikes.

Additional guidance on roof deck sheathing attachment that is written more from the perspective of building a new home can be found in the FEMA coastal construction manual and their recovery fact sheets (FEMA_hgcc_fact18_roof_sheathing_installation). This fact sheet also addresses the issues of gable overhang vulnerability and the need for blocking along the edges of sheathing at ridge vents. In retrofit applications, any desired blocking should be added before re-roofing. The addition of blocking along the edges of the ridge vent is most important for roofs with large gable ends (more than about 8-feet tall) in locations where the design wind speeds are greater than 130 mph.

Not Re-roofing

Enhancing Roof Sheathing Attachment with Sub-floor Adhesive

Uplift resistance can be improved — without removing the roof covering.

Using a caulking gun, apply a 1/4 -inch bead of wood sub-floor adhesive (AFG-01 rated) along the intersection of the roof deck and the roof support element (rafter or truss top chord) on both sides of the structure. Make sure that the adhesive is in contact with both the deck and roof support elements.

Bead of adhesive applied along both sides of the intersection between roof sheathing and rafters or truss top chord.

Similar improvements in uplift can be achieved using blocks of wood with adhesive on two sides.

At places where you have limited access to either side of the roof support, such as the last rafter or truss at the gable end of the house, use 1/2” or 3/4” quarter-round pieces of wood and run them as far along the length of the roof support as you can. Pre-drilling holes for finishing nails before you get up in the attic will make it a lot easier to hold the quarter-round, nail, and adhesive because you can eliminate the need for “the third hand” by inserting the nails into the pre-drilled holes. Apply the adhesive along the two adjacent sides of the quarter round. Press the wood pieces onto the intersection making sure the adhesive is in contact with the deck and roof support elements and tack in place with the finishing nails. You can tell when you have applied enough adhesive if it squeezes out. The nails should not be driven into the deck as they may go through the deck and damage the roof covering. As an alternative, you could clamp the wood pieces in place while the glue sets up.

Note: Be aware that this will permanently attach the roof deck to the top edges of the rafters or trusses. If at a later time one of these pieces of decking is damaged and has to be replaced, the roofer should not just try to pry the deck off the rafter or truss as this will damage the rafter or truss top chord, necessitating expensive repairs. If glue has been used, the roofer will likely have to run a circular saw along the edge of the rafter or truss on either side and remove the sheathing in pieces.

Use of quarter round wood strips to significantly increase strength of the connection.

Attics are typically tight, enclosed areas with poor ventilation. When applying the adhesive, be sure to follow the directions for proper application and ventilation. You can check the labels on the available adhesives and select the one that gives off the least amount of fumes. Nevertheless, a fan that circulates fresh air should be used to help ventilate the work area.

Laboratory tests show that using the wood adhesive can increase the wind uplift resistance of the plywood roof sheathing by as much as three times the old code minimum method of securing the sheathing with 6 penny nails or staples. A variety of AFG-01 rated sub-floor wood adhesives are available at local hardware and building supply stores.

Enhancing Roof Sheathing Attachment with Spray Foam Adhesive

There are also some foam adhesives (usually polyurethane) that can be applied in the attic along the joints between the roof sheathing and the rafters or trusses. These adhesives are widely used in the manufactured housing industry and are different from insulating products. If also sprayed over the joints between the sheathing, these adhesives can help to keep water out if the roof covering is damaged. These systems use special chemicals that are mixed on site and require professional installation. Costs have typically started at about $1.50 per square foot of roof.

An example of foam adhesive sprayed over all joints between sheathing and along intersections between roof sheathing and roof structural members.